1. Field of the Invention
The present invention relates to an objective lens actuator which is mounted on an optical head of an optical disk unit that regenerates information from an information recording medium typified by an optical disk or records information in such a recording medium, and which drives an objective lens.
2. Description of the Background Art
A digital versatile disk (or DVD) can record digital information with a recording density approximately six times as high as a compact disk (or CD). Hence, it is known as an optical disk which can record large-capacity data. In recent years, the amount of information to be recorded in an optical disk has increased, thus seeking for an optical disk with a larger capacity. In order to make the capacity of an optical disk larger, when information is recorded in an optical disk and when the information recorded in the optical disk is regenerated, the light spot formed by a beam of light applied to the optical disk needs making smaller so that the recording density of information becomes higher. If the wavelength of a laser beam from a light source is shortened and the numerical aperture (or NA) of an objective lens is wider, then the light spot becomes smaller. In a DVD, there are used a light source which emits a laser beam with a wavelength of 660 nm and an objective lens which has a numerical aperture (or NA) of 0.6. For example, if a laser which emits a bluish-violet laser beam with a wavelength of 405 nm and an objective lens which has an NA of 0.85, then the recording density becomes five times as high as that of the DVD.
There is an optical disk unit which realizes a high-density recording and regeneration, using a short-wavelength laser beam of a bluish-violet laser. If it has the function of being compatible with an existing optical disk, it becomes more useful as the unit, thus improving its cost performance. In this case, it is difficult to make the working distance as long as that of an objective lens for a DVD or a CD, at the same time that the numerical aperture of an objective lens is widened to 0.85. Hence, in a compatible-type optical disk unit which can execute a recording and a regeneration with a high density, at least one objective lens used for the recording and regeneration of a CD or a DVD and an objective lens for a high-density recording which has a greater numerical aperture than this need to be individually provided.
On the other hand, in an objective lens actuator for an optical disk unit, a magnetic circuit is formed so that it can be driven both in the focusing direction and in the tracking direction. This magnetic circuit has the function of keeping the distance unchanged between an optical disk and an objective lens in the focusing direction and moving the objective lens to a desirable track position in the tracking direction. However, as described above, in an optical disk unit where a plurality of optical disks having a recording density different from each other are compatible, the plurality of optical disks with a mutually-different recording density require their corresponding objective lenses, respectively. Hence, the objective lens actuator has to be configured so that a plurality of objective lenses are mounted on a movable portion and are movable in the focusing direction and in the tracking direction.
In the case where a recording or a regeneration is executed for a plurality of optical disks with a mutually-different recording density, the working distance of each objective lens varies according to the recording density. Thus, the position of a movable portion differs with kept in focus. A prior art discloses that a plurality of objective lenses are mounted on a movable portion, three optical disks have a recording density different from each other, and their positional relations are given with kept in focus. As the prior art, there is the following unit (refer to Japanese Patent Laid-Open No. 2005-302163 specification).
In Japanese Patent Laid-Open No. 2005-302163, on a movable portion are mounted a first objective lens which concentrates a light beam on the recording surface of a Blu-ray disk (or BD) and a second objective lens which concentrates a light beam on the recording surface of a CD or a DVD. The neutral position of a movable block in the focusing direction when an information signal is recorded in or regenerated from the BD is set between the neutral position of the movable block in the focusing direction when an information signal is recorded in or regenerated from the CD and the neutral position of the movable block in the focusing direction when an information signal is recorded in or regenerated from the DVD.
In an objective lens actuator which has the above described configuration according to the prior art, the position of the movable portion with focused on the CD is set in a position closer to the disk than the position of the movable portion with focused on the BD is. In terms of the BD, generally, the numerical aperture (or NA) is wide at about 0.85. Hence, the working distance (or WD) is set at around 0.2 to 0.4 mm, and thus, it is extremely short. On the other hand, in the CD, a rib called a stack rib is formed on the inside of its innermost circumferential data area. Particularly, in a CD-R or a CD-RW, the height of a stack rib is standardized at the maximum 0.4 mm. Thus, if the position of the movable portion with focused on the CD is set in a position closer to the disk than the position of the movable portion with focused on the BD is, then at the time of a recording and a regeneration in the CD, the distance from the disk's surface up to the part of the movable portion which is closest to the disk becomes at least 0.2 to 0.4 mm, or below. In this positional relation, therefore, the stack rib and the movable portion may contact or collide. This raises a disadvantage in that the movable portion comes into contact or collision with the CD, thus making the focusing impossible and hindering the recording and the regeneration in the CD.
In addition, the surface sway of a BD and a DVD is permitted to be 0.3 mm at the maximum in their standard, while the surface sway of a CD is permitted to be 0.5 mm at the maximum. Hence, the movement range necessary for the focusing of the movable portion becomes widest when a recording or a regeneration is executed in the CD. Therefore, among the focusing-operation positions for those three kinds of disks, the focusing-operation position for the CD whose necessary movement range becomes the maximum is supposed to be set in the closest position to the disk. In that case, the total movement range necessary for the three kinds of disks becomes wider as the section where the three movement ranges overlap each other is reduced. This makes it difficult to thin an objective lens actuator.